Millions of Muslims from across the world have gathered in Iraq’s holy city of Karbala to commemorate Arba’een, the 40th day after the martyrdom anniversary of Imam Hussein, the third Shia Imam.

People from at least 60 countries gathered in Karbala on Monday to pay tribute to the grandson of Prophet Muhammad, who was martyred in the desert plains of the city along with his 72 companions some 14 centuries ago.

Imam Hussein and his companions were martyred in a battle with the massive army of Yazid I after refusing allegiance to the tyrant caliph.

Muslim pilgrims are walking on a road towards the holy city of Karbala on November 19, 2016. (Photo by IRNA)

Monday marks the final day of the annual mourning ceremonies for Imam Hussein, which started on the first day of the lunar calendar month of Muharram.

During the first nine days of Muharram, mourning processions were held in anticipation of Ashura, the tenth day, when the Karbala tragedy happened.

Over the past few days, hundreds of thousands from Iraq, Pakistan, India, Iran, Bahrain, Saudi Arabia, Lebanon, Azerbaijan, Turkey and elsewhere have been trekking a main road between Najaf – another holy city in Iraq – and Karbala on foot to show their devotion. The two cities are more than 75 kilometers apart.

An Iraqi policeman stands guard as pilgrims march from the Iraqi capital, Baghdad, to Karbala for the Arba’een ritual on November 15, 2016. (Photo by AP)

Each year, Sunni Muslims and followers of other religious groups such as Christians join the journey to mourn the martyrdom of Imam Hussein.

Some 2 million Iranians are among those attending the ceremony. Arba’een, known as the largest annual pilgrimage in the world, was marked in Iran on Sunday.

Iraqi troops have been deployed to ensure security. The northern and western parts of Iraq have been plagued by gruesome violence ever since Daesh terrorists began a campaign of terror in the country in June 2014.

The terror campaign, however, has not dissuaded pilgrims from around the world from making the journey.